Implantable medical devices (IMDS) are commonly employed in medical therapies such as cardiac rhythm management (CRM), neurological monitoring and therapy, and other diagnostics and monitoring. Such CRM devices may be coupled to a surface of a patient's heart via one or more medical electrical leads. Typically the one or more leads include electrodes for both stimulating the heart and sensing electrical activity of the heart. In order to provide better management of cardiac conditions, one or more leads may also include a physiological sensor such as a transducer. In addition to intracardiac leads, transducers may also be incorporated into leads that are placed in the abdomen, subcutaneously or submuscularly in the thorax or in or around the cranium to measure specific physiologic variables. In many cases, it is desirable that all of the necessary elements, including electrodes and/or transducers, be carried on a single lead body wherein locations of each element along the lead body accommodate proper function to meet the therapeutic objective of the system. Because it is desirable to maintain a small lead diameter while including multiple lead components, it may be preferable that the size of the transducer be as small as possible.
In addition to having a small size, transducers contained within leads must be designed to function well in the environment in which they will be placed. For example, transducers that are exposed to a patient's body are susceptible to corrosion such that integrated circuit chips included in the transducers may degrade over time. To prevent exposure to bodily fluids, transducers may be enclosed in hermetically sealed capsules. The transducer, inside the capsule, is electrically joined to a power source through an electrical feedthrough which may rigidly attach the transducer to the lead. By isolating the transducer, this configuration decreases corrosion, allowing for long-term implantation. However, when the transducers are disposed in or near a patient's heart, heart contractions may cause force to be repeatedly applied to the rigid connection between the transducer and the lead. This undesirable strain may decrease the structural integrity of the transducer over time. Transducers on leads that are placed subcutaneously or intramuscularly would also be subjected to repeated strain due to patient movements. In addition, encasing the transducers in hermetically sealed capsules has the undesirable effect of increasing their size. Some transducers, such as biochemical sensors, cannot be encased in hermetic capsules because they need to interact with the body fluids.
Accordingly, it is desirable to have a relatively simple and inexpensive transducer that has a corrosion resistant configuration and is capable of being implanted into a patient for long periods of time. In addition, it is desirable that the transducer be small to fit within the circumference of an implanted medical device, such as an intracardiac lead. In addition, it is desirable to have a transducer that is configured to be tolerant to strain, such as due to repetitive lead movement caused by cardiac activity.